Cost considerations have prevented the use of titanium compressor wheels in automotive air boost devices. The present invention concerns an economical process for the manufacture of titanium compressor wheels.
Air boost devices (turbochargers, superchargers, electric compressors, etc.) are used to increase combustion air throughput and density, thereby increasing power and responsiveness of internal combustion engines.
The blades of a compressor wheel have a highly complex shape which is design-optimized for (a) drawing air in axially, (b) accelerating this air centrifugally, and (c) discharging air radially outward into the volute-shaped chamber of a compressor housing. In order to accomplish these three distinct functions with maximum efficiency, the blades can be said to have three separate regions.
First, the leading edge of the blade can be described as a sharp pitch helix, adapted for scooping air in and moving air axially. Considering only the leading edge of the blade, the cantilevered or outboard tip travels faster (MPS) than the part closest to the hub, and is generally provided with an even greater pitch angle than the part closest to the hub (see FIG. 1). Thus, the angle of attack of the leading edge of the blade undergoes a twist from lower pitch near the hub to a higher pitch at the outer tip of the leading edge. Further, the leading edge of the blade generally is bowed, and is not planar. Further yet, the leading edge of the blade generally has a xe2x80x9cdipxe2x80x9d near the hub and a xe2x80x9crisexe2x80x9d or convexity along the outer third of the blade tip. These design features are all engineered to enhance the function of drawing air in axially.
Next, in the second or transitional region of the blades, the blades are curved in a manner to change the direction of the airflow from axial to radial, and at the same time to rapidly spin the air centrifugally and accelerate the air to a high velocity, so that when diffused in a volute chamber after leaving the impeller the energy is recovered in the form of increased pressure. Air is trapped in airflow channels defined between the blades, as well as between the inner wall of the compressor wheel housing and the radially enlarged disc-like portion of the hub which defines a floor space, the housing-to-floor spacing narrowing in the direction of air flow.
Finally, in the third region, the blades terminate in a trailing edge, which is designed for propelling air radially out of the compressor wheel. The design of this blade trailing edge is generally complex, provided with (a) a rake angle (angle of surface relative to center line), (b) an angle offset from radial, and/or a back taper or back sweep (which, together with the forward sweep at the leading edge, provides the blade with an overall xe2x80x9cSxe2x80x9d shape). Air expelled in this way has not only high flow, but permits recovery of high pressure over a wide flow range.
Accordingly, functional considerations dictate the complex shape of a compressor wheel.
Recently, tighter regulation of engine exhaust emissions has led to an interest in even higher pressure ratio boosting devices. Current aluminum compressor wheels are not capable of withstanding repeated exposure to higher pressure ratios ( greater than 3.8). While aluminum is a material of choice for compressor wheels due to low weight and low cost, the temperature at the blade tips, and the stresses due to increased centrifugal forces at high RPM, exceed the capability of conventionally employed aluminum alloys. Refinements have been made to aluminum compressor wheels, but due to the inherent limited strength of aluminum, no further significant improvements can be expected. Accordingly, high pressure ratio boost devices have been found in practice to have short life, to be associated with high maintenance cost, and thus have too high a product life cost for widespread acceptance.
Titanium, known for high strength and low weight, might at first seem to be a suitable next generation material. Large titanium compressor wheels have in fact long been used in aircraft jet engines that power aircraft from the B-52B/RB-52B to the F-22. However, titanium is one of the most difficult metals to work with, and currently the cost of production associated with titanium compressor wheels is so high as to limit wide-spread employment of titanium. It is also well known that titanium is highly reactive in the molten state, making it particularly difficult to cast titanium into thin molds without significant mold/metal reaction. This reaction layer must be removed at significant expense. Thin sections aggravate the problem of obtaining a sound casting free of this reaction layer.
The automotive industry is driven by economics. While there is a need for a high performance compressor wheel, it must be capable of being manufactured at reasonable cost. There are presently no known cost-effective manufacturing techniques for manufacturing automobile or truck industry scale titanium compressor wheels having the optimal design described above.
That is, while titanium compressor wheels per se are known, the methods by which they are manufactured are economically prohibitive. For example, it is known to manufacture titanium compressor wheels from solid titanium stock, using computer-aided manufacturing (CAM) equipment, also known as numerically-controlled cutting equipment. However, due to the difficulty of working with titanium, and due to the large amount of material which must be removed, this technique does not come into consideration as an economical means for production of titanium compressor wheels.
Casting techniques are also known, and can be classified into xe2x80x9crubber moldxe2x80x9d techniques and xe2x80x9cinvestment castingxe2x80x9d techniques.
U.S. Pat. No. 6,019,927 (Galliger) entitled xe2x80x9cMethod of Casting a Complex Metal Partxe2x80x9d teaches a method for casting a titanium gas turbine impeller which, though different in shape from a compressor wheel, does have a complex geometry with walls or blades defining undercut spaces. A flexible and resilient positive pattern is made, and the pattern is dipped into a ceramic molding media capable of drying and hardening. The pattern is removed from the media to form a ceramic layer on the flexible pattern, and the layer is coated with sand and air-dried to form a ceramic layer. The dipping, sanding and drying operations are repeated several times to form a multi-layer ceramic shell. The flexible wall pattern is removed from the shell, by partially collapsing with suction if necessary, to form a first ceramic shell mold with a negative cavity defining the part. A second ceramic shell mold is formed on the first shell mold to define the back of the part and a pour passage, and the combined shell molds are fired in a kiln. A high temperature casting material is poured into the shell molds, and after the casting material solidifies, the shell molds are removed by breaking.
It is apparent that the Galliger gas turbine flexible pattern is (a) collapsible and (b) is intended for manufacturing large-dimension gas turbine impellers for jet or turbojet engines. This technique is not suitable for mass-production of automobile scale compressor wheels with thin blades, using a non-collapsing pattern. Galliger does not teach a method which could be adapted to in the automotive industry.
xe2x80x9cInvestment castingxe2x80x9d, on the other hand, involves: (1) making a wax pattern of a hub with cantilevered airfoils, (2) casting a refractory mass about the wax pattern, (3) removing the wax by solvent or thermal means, to form a casting mold, (4) pouring and solidifying the casting, and (5) removing the mold materials.
There are however significant problems associated with the initial step of forming the compressor wheel wax pattern. Whenever a die (comprised of retractable die inserts) is used to cast the wax pattern, the casting die must be opened (die inserts retracted) to release the product. However, since the blades of a compressor wheel have a complex shape as discussed above, the complex geometry of the compressor wheel, with undercut recesses and/or back tapers created by the twist of the individual air foils with compound curves, not to mention dips and humps along the leading edge of the blade, impedes the withdrawal of the several parts of the die (die inserts).
It is know to side-step these problems by fashioning separate molds for each of the wax blades and for the wax hub. The individually formed wax blades and hub can then be assembled and fused to form a wax compressor wheel pattern. However, this creates a new set of problems. It is difficult to assemble a compressor pattern from separate wax parts with the required degree of precisionxe2x80x94including coplanerism of airfoils, proper angle of attack or twist, and equal spacing. Further, stresses are encountered during assembling lead to distortion after removal from the assembly fixture. Finally, this is a labor intensive and thus expensive process. This technique cannot be employed on an industrial scale.
Certainly, titanium compressor wheels would seem desirable over aluminum or steel compressor wheels. Titanium is strong and light-weight, and thus lends itself to producing thin, light-weight compressor wheels which can be driven at high RPM without over-stress due to centrifugal forces.
There is thus a need for a simple and economical method for mass producing titanium compressor wheels, and for the low-cost titanium compressor wheels produced thereby. The method must be capable of reliably and reproducibly producing compressor wheels, without suffering from the prior art problems of dimensional or structural imperfections to which thin blades are particularly susceptible.
The present inventor investigated the problem of how to overcome the above-described technical problems in the manufacture of titanium compressor wheels in order to enable the economical manufacture of titanium compressor wheels. He was initially faced with a number of technical problems.
For example, each individual compressor wheel product must be manufactured with a very high degree of dimensional accuracy. Titanium compressor wheels must be capable of operating at high tip speeds necessary to produce high pressure ratio. Any slight distortion in air foil shape, length and curvature would compromise aerodynamic performance.
Further, errors in blade spacing would generate noise at these high operational speeds. Noise would annoy consumers, and thus noise suppression is an object of the present invention. It is known that aluminum compressor wheels made by casting from re-usable patterns (rubber patterns) often suffer imperfections due to the patterns being non-rigid, and compressor wheels produced thereby often suffer from noise. Thus, the process of the present invention should be capable of producing dimensionally very accurate blade geometries.
Most importantly, the present invention should provide a process with which titanium compressor wheels can be produced significantly more economically than with prior art processes.
The present inventor chose to attempt to develop a method for producing highly accurate positive patterns for use in the xe2x80x9clost waxxe2x80x9d or investment casting technique for forming titanium compressor wheels. In view of the complex shape of a compressor wheel, with undercut recesses and/or back tapers created by the twist of the individual air foils with compound curves, not to mention dips and humps along the leading edge of the blade, it was the conventional wisdom at the time of the invention that it simply would not to be possible to create a xe2x80x9cnon-pullablexe2x80x9d compressor wheel using a solid die.
Nevertheless, the present inventor decided to attempt an entirely novel approach in the manufacture of titanium compressor wheels: to first create a xe2x80x9cnon-pullablexe2x80x9d titanium compressor wheel by a hybrid process involving (1) casting of a wax pattern, followed by (2) machining.
To those working in this art, the approach selected by the present inventor would appear counter-intuitive. That is, if the present invention is driven by economics, logic would dictate that fewer process steps are better than more process steps, and that a manufacturing technique involving both casting and machining would be more labor and equipment intensive than a single technique alone. Thus, those working in this are would not have even considered investigating a hybrid technique as envisioned by the present inventor.
Further yet, it could not be technically predicted that a product of a casting step would be sufficiently accurately dimensioned so as to be able to be simply machined by a xe2x80x9cblindxe2x80x9d tool in a fully automated process to produce a product free of distortion and defect. That is, a casting would have to be located so accurately in, e.g., numerically-controlled cutting equipment that a thin layer of material could be machined from each blade surface.
Further, the inventor had to overcome the problem of how to reliably cast titanium, a metal which is notoriously difficult to cast, particularly in a process of forming a final product having long thin blades.
And finally, even if a marriage of casting and machining could produce a compressor wheel which would be within tolerances required in automotive applications, there remained the important question of whether such a process could be designed to be more economical than the presently available techniques.
After extensive experimentation, the present inventor discovered that the objects of the invention could be achieved, and that a titanium compressor wheel having a non-pullable shape could be economically produced, using a hybrid process in which a positive compressor wheel pattern is first produced by an automated process in a die with solid retractable inserts, but differing from prior art in that the blades of the pattern are modified so as to have the desired shape only to the extent possible with pullable die inserts, i.e., with xe2x80x9cundercutxe2x80x9d or xe2x80x9cbacksweepxe2x80x9d areas being xe2x80x9cfilled inxe2x80x9d, and only to the extent to prevent xe2x80x9cback-lockxe2x80x9d of the die inserts. This compressor wheel pattern is referred to as xe2x80x9cpullablexe2x80x9d since the die inserts can be extracted, leaving the cast wax shape. The cast wax pattern is referred herein to as xe2x80x9cnear net shapexe2x80x9d since only the xe2x80x9cundercutxe2x80x9d or xe2x80x9cbacksweepxe2x80x9d areas, which are filled in as discussed above, need to be machined in the subsequent machining step.
Contributing to the success of the invention is the fact that the filled-in areas contribute dimensional strength during the casting and removal of the wax pattern. Thus, the wax near net shape pattern, and consequently the machined net shape pattern, has a high degree of dimensional trueness as compared to a wax pattern wherein a net shape pattern with very thin blades is cast and pulled.
The near net shape pattern produced as described above could be machined to a xe2x80x9cnon-pullablexe2x80x9d wax pattern shape prior to investment casting.
More preferably, the near net shape (xe2x80x9cpullablexe2x80x9d) pattern is used in the near net shape form in investment casting, and the cast titanium product, having the near net shape, can be machined by conventional techniques to remove the material needed to complete the backsweep and undercut areas of the blades. In this preferred embodiment of the invention, wherein titanium is cast into a mold wherein the near net shape blades are thickened at the xe2x80x9cfilled inxe2x80x9d blade areas, these thickenings coincidentally make the blade much easier to cast than in the case of thin compressor wheel blades. That is, the problems of mold/metal reaction, surface defects, inclusions, etc., for which titanium is notoriously famous, are to a large part overcome by initially casting the blades slightly thicker in accordance with the invention, followed by machining away the xe2x80x9cfilled inxe2x80x9d areas. Accordingly, this embodiment results in a particularly successful casting technique.
Surprisingly, when carried out on an industrial scale, the cost and complexity of machining wax is approximately equal to the cost and complexity of machining titanium. Since the amount of material to be machined in the machining step is small compared to the known technique of, e.g., manufacturing titanium compressor wheels from solid titanium stock using computer-aided manufacturing (CAM) equipment, the process of the present invention is surprisingly economical.
Further yet, it is known that machining of titanium compressor wheels from stock titanium is expensive due to the amount of time required to machine away material and due to tool wear. In accordance with the present invention, since the amount of material being machined away is substantially less than in the case of machining from stock, the tool time and cost are negligible.
Thus, the process according to the present invention is surprisingly economical.
The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood, and so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other compressor wheels for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent structures do not depart from the spirit and scope of the invention as set forth in the appended claims.